drying rates of rubi grapes submitted to chemical ...drying rates of rubi grapes submitted to...

Pesq. agropec. bras., Brasília, v.41, n.3, p.503-509, mar. 2006

Drying rates of Rubi grapes for raisin production 503

Drying rates of Rubi grapes submitted to chemical pretreatmentsfor raisin production

Vânia Regina Nicoletti Telis(1), Vânia Araújo Lourençon(1), Ana Lúcia Gabas(2) and Javier Telis-Romero(1)

(1)Universidade Estadual Paulista Júlio de Mesquita Filho, Dep. de Engenharia e Tecnologia de Alimentos, CEP 15054-000 São José do RioPreto, SP, Brazil. E-mail: [email protected], [email protected] (2)Universidade de São Paulo, Fac. de Zootecnia e Engenharia deAlimentos, Dep. de Engenharia de Alimentos, Caixa Postal 23, CEP 13630-900 Pirassununga, SP, Brazil. E-mail: [email protected]

Abstract – The objective of this work was to determine the most appropriated chemical treatment to be used todry grapes cv. Rubi for raisin production. Drying curves for convective drying with air at 50oC, in a tray drier,were obtained for grapes submitted to chemical pretreatments with different concentrations of potassium carbonateand olive oil, and different dipping times, according to factorial designs. Convective drying curves were alsoobtained for grapes pretreated in aqueous suspensions of soybean lecithin, at varied lecithin concentrationsand dipping times. Page model was adjusted to the experimental drying curves, and the calculated drying timesshowed that the best pretreatment consisted in dipping grapes for 2 minutes in a 5% olive oil and 6% K2CO3

emulsion, at 50oC, which resulted in a drying time close to that of the pretreatment with 2.5% of olive oil, but witha lower consumption of this substance. In addition, the immersion of grapes in an aqueous suspension of 2% soylecithin, at 50oC, for 5 minutes, resulted in a total drying time slightly higher than the most effective pretreatment.

Index terms: Vitis vinifera, drying kinetics, soybean lecithin, olive oil, dipping treatments.

Taxas de secagem de uva Rubi submetida a pré-tratamentos químicospara a produção de passas

Resumo – O objetivo deste trabalho foi selecionar o pré-tratamento químico mais apropriado para a secagem deuvas cv. Rubi para a produção de passas. Foram obtidas curvas de secagem convectiva com ar a 50oC, em umsecador de bandejas, para uvas submetidas a pré-tratamentos químicos com diferentes concentrações de carbo-nato de potássio e azeite de oliva, e diferentes tempos de imersão, de acordo com planejamentos fatoriais.Também foram obtidas curvas de secagem convectiva, para uvas pré-tratadas em suspensões aquosas delecitina de soja, em várias concentrações de lecitina e diferentes tempos de imersão. O modelo de Page foiajustado às curvas de secagem experimental, e os tempos de secagem calculados mostraram que o melhor pré-tratamento consistiu na imersão das uvas por 2 minutos, em uma emulsão de 5% de azeite de oliva e 6% deK2CO3, a 50oC, o que resultou em tempos de secagem próximos aos do pré-tratamento com 2,5% de azeite deoliva, mas com um menor consumo dessa substância. Além disso, a imersão das uvas em uma suspensão aquosade 2% de lecitina de soja, a 50oC, por 5 minutos, resultou em um tempo de secagem total apenas levementesuperior ao do pré-tratamento mais efetivo.

Termos para indexação: Vitis vinifera, cinética de secagem, lecitina de soja, azeite de oliva, pré-tratamentosquímicos.

Introduction

Drying grapes by sun, shade or hot air, is carried outin many countries that produce raisins, usually employedin breakfast cereals, dairy, bakery, confectionery, andnutritional bars (Ramos et al., 2004). Almost all raisinconsumed in Brazil is imported, but the production oftable grapes has been increasing, and there is a great

potential to use surplus or out-of-standard fruits toproduce raisins, reducing losses of fresh fruits andaggregating value to the product (Gabas et al., 1998).

Brazilian production of table grapes can be divided intwo major groups: the fine grapes (Vitis vinifera), mainlyrepresented by cultivars such as Itália and its mutationsRubi, Benitaka and Brasil, in addition to Red Globe andsome seedless cultivars, and the rustic grapes


V.R.N. Telis et al.504

(Vitis labrusca), of which the most famous cultivarsare Isabel and Niagara (Almeida, 2003; Nachtigal, 2003).Three new cultivars of seedless grapes, that are idealfor raisin production, adapted to the tropical climate, wererecently presented (Camargo, 2003).

The waxy cuticle of grape skin controls the rate ofmoisture diffusion through the berries and, in order toaccelerate drying, chemical treatments are applied toremove or modify this cuticle and increase grape skinpermeability to water (Gabas et al., 1999; Pangavhaneet al., 1999). The first dipping mixture consisted of oliveoil and wood ash, but was substituted by an emulsionmade up of a solution of approximately 2.5% food gra-de potassium carbonate (K2CO3) and 1.5–2.0% ofcommercial “grape dipping oil” (Ponting & McBean,1970). This oil is a mixture of about 70% of ethyl estersof C16 and C18 fatty acids with free oleic acid andemulsifiers. The composition of this emulsion hasprobably been achieved by trial and error plus tradition,along with the practice of adding K2CO3 and dippingfor 3 minutes. The active components are the esters andthe K2CO3, which also assists in emulsification byneutralizing the free oleic acid in the oil to form soap(Grncarevic & Lewis, 1976).

Bolin & Stafford (1981) succeeded in improving grapedrying rates with a methyl oleate:K2CO3 (1:1) mixture,whereas increasing carbonate levels increased dryingrates. Gabas et al. (1999) found that increasingconcentration of ethyl oleate between 0 and 3%, in asolution of 2% CaCO3, increased drying rates andresulted in raisins with a less collapsed structure.Different alkaline substances at different concentrationsand temperatures, with or without addition of olive oil,were investigated by Pahlavanzadeh et al. (2001), whoobtained the shortest drying time and best quality raisinsby dipping grapes in a 5% solution of K2CO3, at 42oC.They alerted that skin damage might result frompretreatment with high alkaline solutions. Di Matteoet al. (2000) proposed a physical treatment with super-ficial abrasion of grape peel and found it to be as effectiveas chemical dipping methods in reducing drying times.

Conventional chemical pretreatments are based onthe action of surfactants, present in dipping emulsions; itwould be of interest to test the effectiveness of othersurfactants. Lecithin is a GRAS (generally regarded assafe) emulsifier composed of different phospholipids,glycolipids, carbohydrates and triglycerides and is largelyused in foods (Fennema, 1996). Soy lecithin is a coproductof degumming solvent extracted of soybean oil and could

be a natural alternative surfactant to be applied in grapedrying pretreatments. For Brazilian producers, atreatment based in soy lecithin dispersions would beeconomically advantageous, since olive oil and the othercommercial “dipping oils” are imported ingredients.

Mathematical modeling of the dehydration process isvery useful in design and optimization of dryers. Themost widely used theoretical model that describes dryingkinetics of food materials is the solution of Fick’s secondlaw of diffusion (Nicoleti et al., 2001). Nevertheless, forhigh moisture content materials such as fruits andvegetables, this analytical solution – obtained consideringconstant diffusivity and volume – is not alwaysapplicable, since shrinkage and diffusivity as functionsof moisture content need to be taken into account. Toovercome this problem without making use of complexnumerical solutions, empirical models have been appliedand Page model is frequently used (Parti, 1998).

The objective of this work was to obtain drying cur-ves and to compare drying rates of Rubi grapespretreated in potassium carbonate/olive oil mixtures andin soy lecithin/water dispersions.

Material and Methods

Ripe, fresh grapes (Vitis vinifera) cv. Rubi wereobtained at the local market in São José do Rio Preto, SP,Brazil. The soluble solids content was around 15.8oBrixand moisture content around 0.84 kg water per kilogramof wet material.

Chemical pretreatments consisted of immersing grapeberries, during a pre-determined time interval, in aqueoussuspensions of commercial olive oil and K2CO3, or inaqueous dispersions of commercial unbleached soybeanlecithin, according to two-level factorial designs (Boxet al., 1978) with independent variables and respectivelevels given in Table 1.

Solutions of the desired concentration of K2CO3 wereprepared in distilled water and heated at 50oC, on a hotplate with magnetic stirring. Olive oil was then slowlypoured into this solution, which was kept undercontinuous agitation during dipping of grapes. Aqueousdispersion of lecithin was carried out in a similar manner,with lecithin being directly poured into distilled waterat 50oC under stirring. The temperature of all treatmentswas kept constant at 50oC.

The drying equipment was a pilot scale tray drier,which consisted of three basic sections – an airflowsystem, a drying air heating section and a drying chamber



(Figure 1). The drying compartment consisted offour square metal trays, placed perpendicularly to theairflow. Air was forced through an axial blower, with airvelocity being measured by means of an anemometer.A set of electrical resistances was installed to heat theair. A digital thermometer (Full Gauge, model Penta)measured dry bulb temperature just before air flowingthrough in drying trays. Relative humidity of the ambientair and just of the air before flowing through in dryingchamber was measured by thermo-hygrometers (Lutron,model HT-3004). A ‘honeycomb’ was installed beforethe drying chamber to allow better distribution of airthrough the product.

The berries were placed in a single layer over thetray and inserted into the dryer cabinet, after desiredoperating conditions had been achieved. The traycontaining samples was weighed at regular time intervals.Drying runs were carried out at a constant temperatureof 50oC and air velocity of 1 m s-1. The initial moisturecontent of grapes was determined gravimetrically, usinga vacuum oven (Marconi, model MA-030) at 60oC and110 mm Hg for 48 hours. In order to determine theequilibrium moisture content, drying runs were conducteduntil changes in sample mass were less than 0.1 g.

Page model (equation 1) was fitted to experimentaldrying curves by non-linear regression, using the softwareSatistica v.5.0 (Statsoft, 1995):

)exp(-ktXXXX

M n

eq0

eq =−

−= (1)

in which, M is the dimensionless moisture, X is themoisture content at a certain time t, X0 and Xeq are theinitial and equilibrium moisture contents, respectively, andk and n are the Page drying coefficients, which deter-mine the precise shape of the drying curve. While neither

of these parameters has a direct physical significance,empirical regression equations have been developedrelating both parameters to drying conditions and rawmaterial moisture content (Hossain & Bala, 2002; Wang,2002).

According to Parti (1998), the drying rate for the Pagemodel is given by:

)X-)(X(-kntdtdX

eq1-n= , (2)

and a modified drying coefficient could be defined as:1-nkntk =∗ (3)

If n<1, k* decreases during drying process. Thus,higher values of k could be used to more closelyapproximate the diffusion equation in the initial stagesof drying, without overpredicting drying in the later stages.If n>1, lower values of k might be adopted, withoutsubestimating the drying curve at the end of the process.

The quality of the model adjustment was evaluatedby the correlation coefficient R2, and the root meansquare of residuals, RMS (Lewicki, 2000), calculatedas:

( )[ ]N

MM-M100RMS

2obsestobs∑= (4)

in which, Mobs and Mest are, respectively, the values ofdimensionless moisture observed and estimated by themodel, and N indicates the number of experimental pointsused in the regression procedure.

Table 1. Two level factorial designs for analysis of the effectof different pretreatment conditions on drying rates of grapes.

Figure 1. Schematic diagram of resistive dryer with crossedairflow.



Results and Discussion

Drying curves for grapes, submitted to differentpretreatment emulsions at 50oC, were obtained andcompared with a drying curve determined for untreatedgrapes (Figure 2). A first set of experiments was carriedout according a two-level factorial design with a centralpoint, varying concentrations of olive oil and K2CO3 andkeeping constant the immersion time of 30 seconds(Table 1). The resulting drying curves showed that theapplied pretreatments were effective in accelerating thedrying process, with lower moisture contents at the samedrying time being attained by grapes treated witha 2.5% olive oil and 6% K2CO3 emulsion (Figure 2 A).

Although the adjustment of Page model (equation 1)to experimental drying curves resulted in correlationcoefficients higher than 0.99 (Table 2), there was a lackof fitting in the final stage of drying, where the modelunderpredicted the experimental drying curves (Figu-re 2). Some RMS values (Table 2) were higherthan 25%, which was appointed by Lewicki (2000) asthe maximum limit for acceptance of an adjustmentmodel. It should be considered, however, that in the lastpart of drying curves, the observed moisture contentsthat should be inserted in the denominator of equation (4)are very low, causing RMS to be increased in spite of agood fit to the most part of the curves.

After evaluation of parameters k and n of Pageequation for each drying condition, the model was usedto estimate drying times necessary to obtain raisins withfinal moisture content around 14% in wet basis (Table 2).These calculations were carried out considering the initialmoisture content of 84% (wet basis) – that correspondsto the average value observed in all experimental runs – and the equilibrium moisture content of 1.4% (wet basis)– obtained from the average final moisture contentdetermined in drying runs.

The effect of olive oil and K2CO3 concentrations, withimmersion time of 30 seconds on the necessary dryingtime to attain 14% moisture content (wet basis), may beclearly observed in a surface plot (Figure 3 A). Thestatistical analysis of the results indicated that only theolive oil concentration had a significant effect at 5% levelof significance, with shorter drying times at higher oliveoil concentrations.

Based on the results of the first set of experiments, itwas concluded that an immersion time of 30 secondshad not been enough to cause a great reduction in drying

Figure 2. Drying curves for Rubi grapes dried at 50oC and airvelocity of 1 m s-1. Traces represent the adjustment of Pagemodel (equation 1).

times, which were still very long. Additional tests werethen carried out increasing immersion time of berries in



the mixture of olive oil and K2CO3, and a maximumimmersion time of 2 minutes was established (Table 1),since longer immersion times resulted in burning of grapeskin and formation of cracks. As a consequence, theraisins presented a sticky surface that was visuallyassociated to a bad product quality.

Drying curves determined to samples pretreated inemulsions of 0.5% and 2.5% olive oil and 6% K2CO3,with immersion for 2 minutes, were compared with thoseobtained with immersion time of 30 seconds(Figure 2 B). Also, with the objective of clearlyestablishing the effect of olive oil, a drying run wasconducted after immersion of grapes in a 6% solutionof pure K2CO3. The results showed that olive oil had animportant effect in accelerating drying process, sincegrapes pretreated with pure potassium carbonatepresented the highest moisture contents at the samedrying times. In addition, it was observed that increasingimmersion time in alkaline solutions, during thepretreatment of grapes, was more effective to reducedrying time than increasing olive oil concentrations(Figure 3 B).

Other works have already investigated the influenceof pretreatments with olive oil and K2CO3 on dryingkinetics of grapes, however no results were obtained toallow comparison between different concentrations of

olive oil and K2CO3 (Pangavhane et al., 1999; Vazquezet al., 1999; Pahlavanzadeh et al., 2001; Doymaz & Pala,

Table 2. Drying times (DT) necessary to attainabout 14% moisture (wet basis) and Page model parametersfor Rubi grapes dried at 50oC and air velocity of 1 m s-1,submitted to various chemical pretreatments(1).

Figure 3. Effect of pretreatment conditions on drying timenecessary to produce grapes with final moisture contentof 14% (wet basis). (A) Different olive oil and K2CO3concentrations with immersion time of 30 seconds. at 50oC.(B) Different olive oil concentrations and immersion times ina 6% K2CO3 solution at 50oC. (C) Different lecithinconcentrations and immersion times at 50oC.

(1)k and n: parameters of Page equation; R2: correlation coefficient;RMS: root mean square of residuals.(2)Drying times estimated by the

equation )exp(-ktXXXX

M n

eq0

eq =−−

= , adopting initial moisture content of 84%

(wet basis) and equilibrium moisture content of 1.4% (wet basis).(3)Immersion for 30 seconds. (4)Immersion for 2 minutes. (5)Immersionfor 5 minutes.



2002; Torul & Pehlivan, 2004). Although most of theseworks have used dipping emulsions with 0.4 or 0.5% ofolive oil, and 5 or 7% of K2CO3, there is a greatdiscrepancy between immersion times adopted, whichvaried from 20 seconds to 5 minutes. Drying conditionswere also very different. For a pretreatment with 0.4%olive oil in 7% K2CO3 solution, for 3 minutes at ambienttemperature, and convective drying with air at 60oCand 0.5 m s-1, Pangavhane et al. (1999) obtainedk = 0.0615 h-1 (equation 1), which is somewhat higherthan the value of 0.0533 h-1 found in the present workfor similar pretreatment conditions. The difference couldbe attributed to the longer immersion time of thepretreatment and to the higher drying temperature usedin that work. Doymaz & Pala (2002) observed areduction of 54.2% in the drying time of grapes treatedwith 5% K2CO3 plus 5% olive oil, when comparing withdrying time of grapes without pretreatment at the samedrying temperature (60oC). In this work, a similarreduction of drying time (53.6%) was observed, whencomparing drying of grapes treated with 6% K2CO3 plus5% olive oil, for 2 minutes, with nontreated grapes.

From the results presented above, it was possible toconclude that, when using traditional dipping emulsionsof olive oil and potassium carbonate, the immersion timeduring pretreatment assumes a preponderant role, whichcould be attributed to a low reaction rate of the activecomponents of the emulsion, and the constituents of thewaxy layer of grape skin. During treatment with alkalinesolutions, attention should be drawn to the necessaryimmersion time to increase water vapor permeabilitythrough grape skin and the maximum immersion time toavoid crack formation and burning of grape skin. Thiswould require a great degree of control during processing,which is not always feasible in small processing plants.An alternative of practical interest would be to avoidthe use of alkaline substances during pretreatment, whichcould be possible by using soy lecithin – a naturalsurfactant – in order to eliminate or modify the structureof the grape skin waxy layer.

Drying curves obtained after pretreatments, indispersions of 2 and 4% of soy lecithin with immersiontimes of 2 and 5 minutes (Table 1), showed that lecithinwas effective to accelerate drying process whencomparing with untreated grapes (Figure 2 C). Inaddition, a surface plot of drying times as affected byimmersion time and lecithin concentration showed that,although neither of the variables was significant at5% level, increasing immersion time in the lecithin

suspension was more effective to reduce drying timethan increasing lecithin concentration (Figure 3 C).

Comparison of drying curves of lecithin treated grapeswith that of grapes treated with olive oil and K2CO3,however, demands a more careful analysis. At the initialstage of drying, moisture contents of grapes treated inthe alkaline emulsion were considerably lower than thoseobserved in lecithin treated grapes at the same dryingtimes. Nevertheless, at the end of the process, the dryingcurve of grapes treated with olive oil and K2CO3 assumedan asymptotic behavior, resulting in a very small changeof moisture content with time. At the same time, thedrying curve of grapes immersed for 5 minutes in lecithindispersions showed a continuous decrease in moisturecontent that led to similar total drying times (Table 2).

It was observed that treatments with lecithin resultedin the highest n values, indicating that there was not areduction in drying rates at low moisture contents(Table 2). When pretreatment was carried out in the2.5% olive oil and 6% K2CO3 emulsion for 2 minutes,the lowest value of n was obtained, showing that withthis treatment the drying rate decreased along theprocess. This behavior might be attributed to thedeposition of olive oil on grapes surface, which could bevisually detected by the oily aspect of the raisins resultedfrom this treatment.

This work presented some preliminary results, thatmust be further investigated, in order to determine thebest treatment conditions, as well as to investigate themechanism of action of lecithin on the waxy layer ofgrape skin. It is also important to point out that dryingtimes presented in Table 2 are superestimated, since theywere calculated using Page model fittings, which showeda weak agreement to experimental data at low moisturecontents. Also, the convective drying temperature couldbe increased from 50 to 60oC, in order to acceleratedehydration.

Conclusions

1. The most effective treatment to accelerate dryingprocess is an emulsion of 0.5% of olive oil and 6% ofK2CO3, at 50oC, with immersion time of 2 minutes.

2. Increasing the immersion time of grapes in thechemical treatment reduces drying time.

3. Treatments with lecithin result in the highestn values in Page model, whereas the treatment with



2.5% olive oil and 6% K2CO3 emulsion for 2 minutesresults in the lowest value of n.

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Received on August 8, 2004 and accepted on September 14, 2005

drying rates of rubi grapes submitted to chemical ...drying rates of rubi grapes submitted to...

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